In measuring a pressure space at around 1 Pa using a mass spectrometer, ions collide with a gas molecule while passing through a quadrupole, so that the trajectory course of the ions is disturbed in some cases. This phenomenon is likely to occur under a pressure condition with a short mean free path.
At a pressure of approximately 1E-2 Pa or less, the probability of the collision between ions and a gas in a quadrupole is so low that a decrease in the efficiency of ions reaching an ion detector can be ignored. However, at a pressure of approximately 1E-2 Pa or more, the decrease in the efficiency of ions reaching an ion detector cannot be ignored. Consequently, an ion current value corresponding to a pressure lower than an actual partial pressure is detected at a pressure of approximately 1E-2 Pa or more.
For example, at 1 Pa at which a sputtering process is carried out, the mean free path of an Ar gas is 6.4 mm, and ions are likely to collide with a gas molecule in a quadrupole. Specifically, in a gas analysis at a pressure around 1 Pa, ions are difficult to pass through a quadrupole, so that an amount of ions detected by an ion detector is reduced and is no longer proportional to an amount of ions generated in an ion source. This results in a failure to ensure a linearity between the pressure and the amount of ions detected.
For correcting a measurement value in a mass spectrometry, an apparatus is known which is configured to: measure a total pressure at the time of mass spectrometry; and correct an amount of ions detected with reference to a pre-measured correlation curve between the total pressure and the amount of ions detected. For example, according to the techniques disclosed in Patent Documents 1 and 2, a total pressure is measured by measuring ions generated in an ion source of a mass spectrometer without passing the ions through a mass analyzer. Moreover, in the technique disclosed in Patent Document 3, a total pressure is calculated from a detection value (equivalent to an ion current value corresponding to a mass-to-charge ratio at which no peak appears in Patent Document 3) of a photoelectron generated as a result of irradiating an ion detector disposed at the next stage of a mass analyzer with vacuum ultraviolet light generated in an ionization chamber when a measurement gas is ionized.